help me with RS-485 to RS-232 converter, please !

[electronproton]

rs485 data bounce

Hi all..
I design a network using 485 protocol.
it connected to computer using rs485 to rs232 converter..
at first i use short cable (10m) with termination resistor 120 ohm.. but it didn't work.. when i didn't use the termination resistor it work well.
but when i change to 100m cable it didn't work either using termination resistor or not..
plz help me..
thx

 

[laktronics]

Re: need help with RS-485

Hi,
Did you connect the ground pins of the transmitter board and receiver board? Do you have a proper supply for the drivers and receivers, or using a parasitic supply derived from some signals? Please post your circuit for better comments.
Regards,
Laktronics

 

[electronproton]

Re: need help with RS-485

no problem with the supply.. coz it works for 10-20 m cable but not in even 30 m cable!!
i don't know what's wrong.. shouldn't 485 support up to 1000m++?

 

[VVV]

Re: need help with RS-485

I agree with Laktronics.
I think the converter you are using actually derives its power from an RS-232 signal on the PC.
There are some models that allow you to use an external supply. if that is the case with yours, then use it with an external P/S and it should work.

And, yes, check the ground difference between the two circuits. If it's too high, the communication will fail.

 

[banjo]

need help with RS-485

Are you using twisted pair cables? Remember that RS-485 is a differential pair system. For long runs you need twisted pair wires and you need to insure that the pairs are properly matched.
In a former job we communicated with RS-485 across large distances. On the eastern coast of the US, the unions are very strong. We were prohibited from running wires in installations ourselves and had to hire union contractors. The used the correct wires, but the swapped the one wire in each pair in a remove junction box. It took an long time to figure that one out!

Also, did you include the weak pullup and pulldown resistors to hold the lines in a idle state? If you do not do that, then noise can corrupt the receiving UART's datastream.

 

[parthaarathi]

Re: need help with RS-485

Hi,

Also see whether ur rs-485 driver getting sufficient current from the power supply.
Because RS-485 driver require large amount of current(~ 70ma max).

 

[auto_mitch]

Re: need help with RS-485

At the following link there is an article for Designing RS-485 Circuits.

 

[electronproton]

need help with RS-485

I believe there's no short circuit...
I'm using external power supply for both the converter and the 485, connected to 7805..
and i'm using 9600 bps baud rate..
When the device transmit data to the converter (PC) there's no problem.. but when the PC transmit data, the data error, the device receive data but not the same data..

 

[IanP]

Re: need help with RS-485

Try to connect bias resistors, as shown in this example (see J3 and J4):
https://www.rs485.com/pdffiles/converters/rsfc44t.pdf

Regards,
IanP

 

[electronproton]

Re: need help with RS-485

thx for the ideas.. i'll try it tonight.. thx all

Added after 1 hours 37 minutes:

Hi all.. I discover that the power supply is not the problem coz i just tried it using laboratory power supply and it still doesn't work with long cable.

my design is like this:

PC - 485/232 Converter ---------------- 485 device

I discover that:
1. the device has no problem transmit the data to the PC either short or long cable.
2. the PC has no problem transmit data in SHORT cable.
3. using LONG cable... if the PC send data, the device receive different data and somehow the PC receive a data too.. so i think the data "bounce" back?

 

[FvM]

Re: need help with RS-485

Hello,

it's been already said: Without knowing your converter circuit (or the type in case of a commercial product) , it's impossible to see what's going on there. As a remark: It's normal operation, that RS485 "bounces back" the sent data unless the read operation from RS485 isn't supressed during send. This operation is among the interesting details that make the difference between existing RS485 converter variants.

I add a very simply converter circuit I was using before. It is intended to be supplied from 10-30 V DC, but misses the said weak pull-up/-down resistors.

Regards,
Frank

 

[jitendrabatham]

Re: need help with RS-485

Hi,

After reading all the post reply,i should advice to use good quality cable for communication,

This cable is named as "CAT-5" cable with twisted pair.

Try it & reply.




Regards,

Jitendra Kumar,
Pune.